This section is intended to provide background information to facilitate a better understanding of the various aspects of the described embodiments. Accordingly, it should be understood that these statements are to be read in this light and not as admissions of prior art.
In some offshore drilling operations, a wellhead at the sea floor is positioned at the upper end of the subterranean wellbore lined with casing; a blowout preventer (BOP) stack is mounted to the wellhead; and a lower marine riser package (LMRP) is mounted to the BOP stack. The upper end of the LMRP may include a flex joint coupled to the lower end of a drilling riser that extends upward to a drilling vessel at the sea surface. A drill string is hung from the drilling vessel through the drilling riser, the LMRP, the BOP stack, and the wellhead into the wellbore.
During drilling operations, drilling fluid, or mud, is pumped from the sea surface down the drill string, and returns up the annulus around the drill string. In the event of a rapid invasion of formation fluid into the annulus, commonly known as a “kick”, the BOP stack and/or LMRP may actuate to help seal the annulus and control the fluid pressure in the wellbore. In particular, the BOP stack and LMRP include closure members, or cavities, designed to help seal the wellbore and prevent the release of high-pressure formation fluids from the wellbore. Thus, the BOP stack and LMRP function as pressure control devices.
Pressure accumulators provide a pressurized working fluid for the control and operation of subsea equipment, such as the BOP stack. In particular, pressure accumulators are used to set the hydraulic timing in triggering the various BOPs in the BOP stack to seal the wellbore, especially in a deadman trigger sequence when the drilling riser is removed from the BOP stack. However, pressure accumulators have fixed volumes, which controls the minimum time delay produced by the accumulator. In particular, this minimum time delay can be affected by various factors, such as ambient temperature, hydrostatic pressure, as well as factors related to hoses, tubing, valves, or other hydraulic devices in communication with the accumulator (e.g., movement, crimps, clogging), etc. These factors can even arise after the accumulators are deployed at a subsea location. One approach to adjust the time delay of a pressure accumulator is to reduce the flow rate of fluid into it using a flow control valve. Thus, the time delay produced by the accumulator can be increased, but not decreased. However, fine adjustment of flow rate is a challenge with flow control valves.